Relaxation oscillator



March 18, 1941. F. E. BLOUNT ET AL 2,235,667

RELAXATION OSCILLATQR Filed Dec. 16,1938

TON OUTPUT F I G. 2 m

AVL

L 60 LEM. i a B F ILLEO HI M E E. BLOU/VT 'A. K SCHE VC/f A T TORA/EV Patented Mar. 18, 1941 PATENT OFFICE RELAXATION oscirm-rroa "Frank E. Blount, Cedar Grove, and Alfred K.

Schenck, East Orange, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 16, 1933, Serial No. 246,174

4 Claims.

This invention relates to means for producing a succession of oscillating impulses which occur at cyclically varying intervals, that is, impulses which recur at a gradually increasing rate until they reach a predetermined maximum rate and then gradually decrease to a predetermined minimum rate, which cycle of increase and decrease is periodically repeated. Such rapidly recurring impulses, for example those which start recurring at a rate of the order of two hundred per second and gradually increase to four hundred per secend and then decrease to the two hundred rate again, which cycleof increase and decrease is periodically repeated, at a rate of the order of one per second, for example, are useful as tone signals in telephony,

A feature of the inventionresides in applying an exponentially varying direct current input voltage to a relaxation oscillator employing a gas-filled discharge device whereby, as the input voltage increases, the frequency of recurrence of the impulses increases correspondingly and as the voltage decreases the recurrence rate correspondingly decreases.

The invention will be understood from the following description and accompanying drawing which illustrates one embodiment thereof, Fig. l of which illustratesthe arrangement of the apparatus and circuit to achieve the desired results, and Fig. 2 graphically shows the cyclically varying frequency of recurrence of the impulses.

In the drawing (Fig. 1) 3 is a source of direct current, which in practice may be of the order of 150 volts, connected by means of a switch S through a resistance RI, of the order of 500,000 ohms, and an interrupter I, operating at an approximate rate of sixty interruptions per minute, across a condenser A having a capacity of the order of 1 microfarad. Connected across the condenser A, in series with a second high resistance R2, also of the high order of 500,000 ohms, is a gas-filled glow discharge device G and connected in parallel with the device G is a series circuit including the primary winding of a transformer T and a second condenser B of a relatively small capacity with respect to condenser A, i. e., of the order of .007 microfarad. The resistance of the primary winding of transformer T may in practice be of the order of 500 ohms, and the second ary of the order of 20 ohms.

The operation of the circuit arrangement is as follows: It will be assumed that interrupter I is in operation and switch S is closed, whereupon condensers A and B start to charge in parallel with the source 3. When the charge on condenser B reaches the breakdown potential of the device G, a glow discharge will occur between the electrodes whereupon the circuit including device G, winding I and condenser B will oscillate in a well-known manner and generate in the 5 secondary winding 2 of the transformer a Wave of the characteristic shown in Fig. 2. When the potential across condenser B drops below the value necessary to sustain the glow discharge, the arc will be extinguished and condenser B will again start to charge. When the glow discharge in device G was established the effective rate of charge on condenser A was reduced but it continued to increase its charge, due to the high resistance R1, although at a slower rate, during 1 the period condenser B was dissipating its charge through the oscillating'circuit. When the are be tween the electrodes of device G- is extinguished and condenser B again starts to build up its charge, condenser A is at a higher potential than it was previously whenthe device G fired and, consequently, condenser B will arrive at the critical potential necessary to break down the device G sooner than before. When this point is reached the device G again fires and another group of oscillations is generated in the winding 2 of the transformer. This cycle of operations continues with condenser B reaching the breakdown potentials of device G at a gradually increasing rate, as shown in Fig. 2, until interrupter I opens its contact, thereby disconnecting the source 3.

The next time the charge on condenser B drops below the sustaining voltage of the arc of device G, due to the fact that the source 3 is disconnected, the only remaining source of charging current for condenser B is condenser A which has by this time attained a relatively high charge which is now effective to recharge condenser B at a rate which may be only slightly less than the next preceding charging time which was aided by the source 3. Condenser B upon attaining the critical potential discharges as before and again recharges from condenser A. This drain on condenser A gradually reduces its charge so that each successive charge of condenser E is attained at a progressively slower rate so that successive impulses generated in winding 2 of the transformer occur at a correspondingly slower rate until the interrupter contacts are again closed whereupon condenser A again commences to increase instead of lose its charge with each successive charge of condenser B, whereupon the rate with which successive trains of oscillations are generated in winding 2 again start to increase at the previous rate.

Ihe foregoing cycle of increase and decrease in the frequency of occurrence of the trains of oscillations continues until the switch S is opened, whereupon the oscillations cease when condenser B no longer receives a suflicient charge to fire the device G.

It should be noted that oscillations take place in the circuit B, I, G due to the difference between the breakdown and minimum sustaining voltage of the device G (see Kock, Electronics. March 1935, page 92) and if the inductance of winding l is small the frequency of oscillation is substantially determined by the other constants of the circuit, 1. e., the energizing voltage, the resistance in series, and the capacity of condenser B. Therefore, any desired frequency of oscillation can be obtained by the proper selection of these constants Without the inclusion of inductance.

In the arrangement of the present invention the introduction of inductance by including the primary I of the transformer is merely incidental and in designing the circuit it is only-necessary to avoid employing a transformer winding which will make the circuit resonant to a frequency lower than desired, which oscillations obviously should be higher than the rate of recurrence of the impulses.

In the present case the rate of recurrence of the impulses ranges from 200-400 times a second and the frequency of the impulse is of the general order of 1000 cycles,

Audible frequencies occurring at cyclically varying rates, as described, are suitable as tone signals in telephony and may find extensive use.

It will be understood that while a two-element discharge is shown and described, a three-element device, well known in the art, can be substituted therefor without departing from the spirit of the invention.

What is claimed is:

1. In a relaxation oscillator for producing successive groups of oscillations recurring at cyclically varying intervals, a gas-filled discharge device having a cathode and an anode, an output circuit therefor including a condenser and a transformer winding serially connected across said anode and cathode and an input circuit therefor serially including said anode and cathode, a high resistance and a periodically interrupted source of direct current, and a second condenser in shunt to said interrupted source.

2. In a generator for producing successive groups of oscillations recurring at cyclically varying intervals, a gas-filled discharge device having a pair of closely spaced electrodes, a source of periodically interrupted direct current and a circuit serially including a high resistance connecting said interrupted source across said electrodes, a condenser in shunt to said high resistance and said electrodes and a second circuit also connected across said electrodes and serially including a second condenser and a work circuit.

3. A generator for producing successive groups of oscillaitons recurring at cyclically varying intervals comprising a periodically interrupted unidirectional source of potential, a condenser shunting said interrupted source and charged thereby, a high resistance effectively in series with said interrupted source and condenser, a glow discharge tube and a second high resistance in series across said condenser, and a work circuit serially including a second condenser across said tube, said second condenser being of much smaller relative capacity than the first condenser whereby when the circuit of said source is completed the second condenser will charge to the firing voltage of the tube and discharge therethrough to produce successive groups of constant frequency oscillations in the work circuit recurring at an increasing rate over a predetermined range as long as the source is connected and when the source is interrupted U to continue to charge from the first condenser and discharge through the tube to produce successive groups of oscillations of the same frequency but recurring at a decreasing rate until the first condenser is discharged to a voltage too low to charge the second condenser to the firing voltage of the tube.

4. In a relaxation oscillator, an oscillating circuit comprising a glow discharge tube and a condenser in series, means for causing said tube to alternately fire and extinguish at cyclically varying intervals to produce successive groups of constant frequency oscillations in said oscillating circuit recurring at the rate at which said tube fires, said means comprising a high resistance circuit including a periodically interrupted source of direct current shunted by a second condenser, said high resistance circuit being connected in parallel with one of said oscillating circuit units.

FRANK E. BLOUNT. ALFRED K. SCI-IENCK. 

